The term "wet adhesion" is used to describe the ability of a paint coating to retain its adhesive bond to a substrate under wet conditions. Good wet adhesion is well known in oil-based paints, but water-based paints tend to lose adhesion when wet. This inherent deficiency limits the usefulness of latex paints, especially vinyl-acrylic and all-acrylic latexes which are otherwise very attractive as paint vehicles.
Paints intended for outdoor use are frequently exposed to moisture due to rain and high humidity. Similar conditions are also encountered by interior paints used in bathrooms and kitchens. Furthermore, resistance to washing and abrasive scrubbing is also an important requirement for painted surfaces which become soiled and must be cleaned.
The property of wet adhesion to pre-coated substrates has been a highly desired property for water-borne acrylic or vinyl-acrylic polymers in architectural coatings markets. Amines were first used for wet adhesion in the early 1960's. U.S. Pat. No. 3,261,796 showed that carboxylic acid-containing latexes could be functionalized with amines by reacting the carboxylic acid group with the very toxic compounds, aziridines. ##STR1##
U.S. Pat. No. 3,356,627 teaches that dialkyl and monoalkylaminoethyl methacrylates and acrylates impart wet adhesion to coatings: ##STR2##
U.S. Pat. No. 4,421,889 describes the use of paints with improved wet adhesion using polymers containing derivatives having the acetoacetate functionality. In particular, this reference discloses the use of acetoacetoxyethyl methacrylate (AAEM) in polymers to improve wet adhesion of paints after a drying period of 24 hours. This reference does not teach the use of amino methacrylates. This reference further teaches that dispersion paints containing amino groups have a tendency to become yellow, and that their presence only slightly improves the wet adhesion of paints.
U.S. Pat. No. 5,326,814 teaches the use of ethylenureido-containing monomers in combination with acetoacetoxy-containing monomers and/or cyanoacetoxy-containing monomers, in combination with copolymerizable surfactants, olefinic carboxylic acid monomers, and a combination of hard and soft monomers.
When a paint is applied to a substrate, it is susceptible to losing its adhesion if high moisture conditions are present prior to the development of a wet adhesion property. The time it takes for coatings to develop wet adhesion is as important as the final wet adhesion property itself. We have found that paints prepared from polymers which contain, for example, polymerized acetoacetoxyethyl methacrylate (AAEM) and t-butylaminoethyl methacrylate (t-BAMA) monomers in addition to other ethylenically unsaturated monomers develop outstanding wet adhesion within five hours. Polymers containing only AAEM as a wet adhesion monomer produced no detectable wet adhesion within 5 hours, and a very slight improvement in wet adhesion after 24 hours. Polymers containing only amino-functional monomers, e.g., t-BAMA, as a wet adhesion monomer produced coatings with some wet adhesion in 5 hours, but this composition was found far inferior to latexes containing polymers prepared from both AAEM and t-BAMA. When formulated into paints, and stored for one year before application, compositions containing AAEM and t-BAMA continued to give wet adhesion.